The study of human behavior as it relates to biology and physical biochemical states. It focuses on the study of how & why the body affects the mind, and how & why the mind affects the body.
Human behavioral biology is probably best described as an aspect of human behavioral ecology (HBE) which studies human behavior in the context of all of its influencing factors - biology, society, evolution, and more.
Courtesy of Stanford professor Robert Sapolsky.
Stanford Professor Robert Sapolsky's course playlist.